Adding Apple Watch to calls on other devices?

You should be good if your iPhone and Apple Watch are somewhere inside the same 3-4 bedroom sized home.

Also if they are both on the same WiFi home network, the Apple Watch can use WiFi, and if your iPhone carrier supports WiFi calling and you have enable WiFi calling on your iPhone, then your Apple Watch can use WiFi calling.

This morning my Wife answered a call on her GPS only Apple Watch from the 2nd floor bedroom while her iPhone was in the kitchen (50-60 feet away going through 2x4 drywall covered walls and crossing one floor.

You don't. The Apple Watch will automatically receive calls from the phone it's paired to.

There are a number of things the watch can do without being in range of the iPhone if it's connected to WiFi:

Use your Apple Watch without your iPhone nearby - Apple Support

Depending on your carrier, that may include phone calls.

Realistically bluetooth has range of about 30 feet,a bit less if there are thick walls etc.

Your welcome.